Accurate control of the orientation of polymeric structure in thin layers is desired to maximize their mechanical, chemical, and optical properties. While orientation can be performed through mechanical means, it is often more desirable to orient the structure during the polymerization process, particularly if the process involves polymerizing the system into a specific final shape, where further mechanical manipulation is unfeasible.
Biomedical components often require oriented structures. Tendons, for example, contain highly oriented collagen fibrils, and the spinal intervertebral disc is composed mainly of oriented crystalline collagen fibrils and amorphous hydrophilic proteoglycan. The prevalence of oriented collagen in the human body makes formation of highly oriented layers of this polymer a desired goal in modeling of tissue structure. Collagen is a biopolymer and protein, and found in the structure of tendons, skin, bones and blood vessels. Randomly oriented collagen materials are weak, and degrade quickly when exposed to mechanical stress. Soluble collagen is derived from animal tissue, and can be obtained in a monomeric form. The collagen monomer will polymerize, termed “fibrillogenesis,” to form a gel-like structure. This process is achieved when the collagen is in a solution of specific pH, temperature, and ionic strength. The polymerization occurs as a self-assembly process, producing native-type collagen fibers. The fibers grow into a porous gel matrix.
The collagen molecule is a rod-like structure in the unaggregated state, composed of three peptide chains intertwined to form a triple helix. Improvements over randomly oriented gels in mechanical and optical properties can be realized if the aggregated molecules can be coerced into an oriented, or aligned, state. Cast films of collagen can form a randomly oriented gel structure, which lacks desired mechanical and optical properties. It is desirable to give a structured order for optical transparency in corneal replacements.
Collagen fiber orientation techniques have included mechanical deformation of already gelled matrices and laminar flow of a gelling matrix. These approaches yield gelled layers hundreds of microns thick.